Business and consumers use a wide array of wireless devices, including cell phones, wireless local area network (LAN) cards, global positioning system (GPS) devices, electronic organizers equipped with wireless modems, and the like. The increased demand for wireless communication devices has created a corresponding demand for technical improvements to such devices. Generally speaking, more and more of the components of conventional radio receivers and transmitters are being fabricated in a single integrated circuit (IC) package. In particular, single chip radios with no off-chip channel filters are very popular. Direct conversion receivers and software-defined radios are being developed in order to simplify single chip designs and to make each design suitable for as many applications as possible.
In radio transceivers, mixers are widely used for translating signal frequencies. Mixing is an essential circuit process for all radio designs. It must exist regardless of the radio architecture. Conventional superheterodyne radios employ many mixers for translating radio frequency signals (RF) to intermediate frequency signals (IF) for signal amplification and filtering. Modern direct-conversion radios still require one mixing function for demodulating RF signals into baseband signals. The mixing process requires a local oscillator (LO). Depending on the type of radio architecture, the LO frequency can be higher, lower or equivalent to the RF frequency. The frequency difference equals either the IF or baseband frequencies.
There are two principal types of mixers: 1) the analog-multiplying mixer and 2) the switching mixer. An analog-multiplying mixer performs the basic multiplying function and the output is simply the product of two input signals. This method requires a linear multiplier to yield good spurious signal rejection performance. In modern integrated receivers, a switching mixer is more popular because of its simpler design, better signal linearity, and superior dynamic range. However, a major drawback of switching mixers is that the mixer output responds to odd harmonic frequencies of the local oscillator (LO) signal. This is an intrinsic characteristic of the design because the superior linearity relies on hard switching of the signal paths. Hard switching requires a harmonic-rich square wave signal source, which is the source of the unwanted responses.
Traditionally, multiple high-Q band-select filters have been utilized to provide the majority of suppression at these out-of-band harmonic frequencies. Special PCB pattern designs are also common practices to further enhance the suppression. Some designs even take advantage of the high frequency roll-off characteristic of the front-end low-noise amplifier (LNA) to further suppress the unwanted responses. In designing modern radio transceivers with stringent out-of-band specifications, the total available suppression from existing methods is always found to be inadequate. This condition worsens when higher transmitting frequencies are employed in newer radio standards, since common band-select filters exhibit poor rejection at higher frequencies. The advent of single-chip radios also fuels the degree of this problem. Ultimately, inadequate out-of-band harmonic suppression becomes a major roadblock in manufacturing high performance radios.
Therefore, there is a need in the art for an improved RF receiver architecture. In particular, there is a need in the art for an RF receiver architecture that has a simple design, improved signal linearity, and superior dynamic range. More particularly, there is a need for an RF receiver that provides a high degree of out-of-band harmonic suppression.